Fluoride Adsorption Mechanism On Clay Minerals And Aluminum Sulfate Impregnated Activated Alumina Granule Adsorption Studies

Kaolinite (K), Montmorillonite (M), Chlorite (Ch), Illite (I) are four representative clay minerals in nature which are selected as research objects. This research investigates fluoride (F-) adsorption mechanism onto four kinds of clay minerals under different pH, F- concentration and reaction time, and studies superficial layers morphology of F as well as elements composition by X-ray photoelectron spectroscopy (XPS) analysis. In high F- concentration(Co) solution (5-1000 mg/L), F-adsorption amount(QF), hydroxyl release amount(HRA) of clay minerals, equilibrium F- concentration(CF) and equilibrium pH increase with increasing Co, especially remarkably when above 50 mg/L. And by modifying and maintaining solution pH at 6, F- adsorption amount of clay minerals is higher than that without pH regulation. Various adsorption data and F- binding energy of superficial layers (FSBE) detected by XPS indicate that at Co less than 5-100 mg/L clay minerals firstly adsorb H+ to protonate aluminum active hydroxyl sites (AAHS) in superficial layers, and react with F-; and as Co gradually increases clay minerals can adsorb F- directly together with the adsorption of some cations; and at Co greater than 100 mg/L F- precipitates in the forms of cryolite or AIF3. The type of new minerals formed in superficial layers in high Co solution depends on Al3+ concentration (CAl)-cryolite at CAl greater than 10-11.94mol/L, AIF3 at CAl less than 10-11.94mol/L. In low CO (0.3-1.5 mg/L) solution proton transfer occurs to make CF decrease continuously with passing of time.The second chapter research the defluoridation capacity of aluminum sulfate impregnated alumina granule (AIAA) in treating high fluoride water. Batch experiments study the influence of operational factors on fluoride adsorption by varying adsorption amount (2-40 g/L), fluoride concentration (2-100 mg/L), pH (4-10), temperature (11-33℃), time. Column experiments study the influence of fluoride concentration (10-50 mg/L), flow rate (2-10 mL/min), filler height (10-20 cm) on defluoridation capacity of AIAA column. At 11-33℃, pH 4-10 and adsorbents amount 20 g/L, the efficiency of AIAA treating 10 mg/L fluoride solution in 3 h is more than 90%, and the existence of a little amount of anions do not influence fluoride adsorption. The study of adsorption equilibrium fitting models illustrate that Langamuir model linear fitting is the optimistic fitting method when AIAA in low fluoride concentration solution; while Freundlich model non-linear fitting is the optimistic fitting method when AIAA in high fluoride concentration solution. Thermodynamic studies show that chemical adsorption plays the dominant role of fluoride adsorption by AIAA; kinetic studies show that adsorption rate is controlled by surface diffusion process and adsorption reaction tallies with pseudo-second order kinetics. In column experiments, lowering flow rate or increasing filler height can improve defluoridation efficiency. In regeneration process,0.01 M NaOH has the best regenerated effect which can regenerate saturated AIAA completely by three times renewing.